1. Field of the Invention
This invention relates generally to medical apparatus for measuring characteristics of an animal heart, and more particularly to a diagnostic catherer to be used with appropriate equipment whereby cardiac outputs can be monitored on a beat-by-beat basis over a prolonged period of time.
2. Discussion of the Prior Art
In assessing cardiac performance and in diagnosing heart abnormalities, an important parameter to be observed is cardiac output, which is generally measured in terms of liters-per-minute and which corresponds to the heart's stroke volume multiplied by heart rate. For example, following the occurrence of a cardiac infact, the attending cardiologist may want to assess the amount of damage in terms of the heart's ability to pump blood. Also, when certain drugs are administered, the attending physician will want to monitor the effects of such drugs on cardiac performance.
Various methods are known in the art for measuring cardiac output. A common approach has been the use of a thermal dilution technique in which a catherer is used to inject cold saline solution into the heart and further means are provided on the catheter for sensing temperature at a point exterior to the heart, usually in the pulmonary outflow tract. By its very nature, the procedure can only be used on an intermittent basis at relatively widely spaced intervals. The thermal dilution technique is not capable of providing real-time data on a beat-by-beat basis.
More recently, researchers have found a way to measure stroke volume through the use of a technique called impedance pleythsmography. Here, a catherer having a plurality of surface electrodes is inserted into the right ventricle and an AC voltage is applied across one pair of space-apart surface electrodes, which may be referred to as the drive pair. At the same time, voltage signals are sensed at intermediate pairs of sensing electrodes and it is found that these signals are proportional to the impedance between the sensing electrodes, which impedance is a function of the quantity of blood contained in the heart chamber between the sensing electrodes in question. The beating action of the heart thus modulates the applied AC carrier signal and, using available signal processing techniques, the modulating signal can be removed from the carrier and it is found to be proportional to stroke volume.
Those readers desiring more information on the impedance pleythsmography technique are referred to the co-pending application of Rodney Salo et al, application Ser. No. 362,903, filed Mar. 29, 1982. Now U.S. Pat. No. 4,686,987 which is assigned to the assignee of the instant application, as well as to the published references cited therein.
The present invention is concerned with the design of a special-purpose catherer which has been developed to facilitate the real-time monitoring of stroke volume and, therefore, cardiac outputs using the impedance plethsmography technique. Specifically, the catheter has been designed to facilitate the positioning of the driving and sensing electrode pairs within the ventricular chamber of the heart in such an orientation that accurate readings can be insured. The catherer is designed so that it will be disposed in the right ventricle with a drive electrode located in the apex and with another drive electrode being located near the pulmonic valve and with the intermediate sensing electrodes spaced away from the endocardial tissue improving the quality of the intracardiac impedance signals and minimizing cardiac induced PVCs. The construction thus reduces the risk of catheter-induced arrhythmias and allows the catheter to remain in place for prolonged periods while providing the physical placement of electrodes necessary for accurate cardiac output determinations by impedance plethysmography.